1. Field of the Invention
The present invention relates generally to the field of three-dimensional imaging and analysis, and more particularly, to a system, method and computer readable medium for adjusting stereo X-ray images in order to correct errors and distortion resulting from illumination disparities, depth plane curvature, keystone distortion, and depth range.
2. Description of Background Art
For both real-world and computer-generated imaging applications, there is a growing need for display techniques that enable determination of relative spatial locations between objects in an image. Once the spatial relations are established, a user may move through the display space and manipulate objects easily and accurately. There are a variety of techniques for collecting as much information as possible about the object by analyzing the image.
One area of interest pertains to computer object recognition of binocular stereo, wherein three-dimensional shapes of objects of interest are created from images taken of a generally static scene by one or more imaging devices disposed at two different positions. The application of the binocular stereo to X-ray imaging entails projecting images onto a viewing device. With X-ray stereoscopic imaging, it will be apparent that the pair of X-ray images may be made using one X-ray source shifted by a distance, or by a pair of X-ray sources. In certain fields of use, the images are generally viewed on radiographs. A benefit of stereoscopic X-ray imaging is that it is an effective method for obtaining three-dimensional spatial information from two-dimensional X-ray images without the need for tomographic reconstruction. This much-needed information is missed in many conventional X-ray diagnostic and interventional procedures, e.g., in orthopedic or chest X-ray imaging. Binocular X-ray imaging has generally not been easy to implement because there are no visible surfaces on the radiograph that may be recreated in a three-dimensional image, as compared to reflective photography of images. Rather, with X-ray imaging, information about different objects are typically located within the same areas of the X-ray image.
Numerous algorithms for image matching have been proposed. They can roughly be classified into two categories. In the first category, the algorithms attempt to correlate the gray levels of image patches, assuming that the image patches present some similarity. In the second category, the algorithms first extract salient primitives or feature points from the images, such as edge segments or contours, and then match these features in two views. These methods are relatively fast, because only small subsets of the image pixels are used, but often fail because the chosen primitives cannot be reliably detected in the images.
Moreover, conventional methods of X-ray stereoscopic imaging have been unsatisfactory due to the insufficient manner of addressing the differences in illumination (i.e., brightness) amongst stereo image pairs. These differences may result based on the position of the X-ray sources, which project the images of the object upon a viewing device from different angles. In other instances, when the same object is exposed to two different X-ray sources or by one source shifted horizontally or temporally, the illumination of the scene changes depending upon the conditions. Such differences in illumination of the stereo images is problematic because, for example, the right part of the right image may be darker than the right part of the left image. More specifically, the illumination differences result in inconsistent brightness and contrast of the same area amongst the image pair. In order to be able to perform a matching analysis on the images and to create a three-dimensional spatial representation of the object from the pair of two-dimensional projection X-ray images, without the need for tomographic reconstruction, these illumination errors must be substantially reduced or eliminated. These illumination errors also lead to difficulties in observing images in stereo, in visual comparison of two images taken at different times, and in mathematical analysis such as stereo matching or image subtraction. Since it is often desirable to compare two images of the object taken at different times, or with different X-ray sources, conventional methods and systems insufficiently address the situation where the illumination in each image may be different, requiring correction before appropriate analysis can take place.
Conventional techniques that adjust the brightness and contrast of two images using a histogram adjustment method are problematic in that the equalization adjustment is made for the whole image, so the differences in illumination within the images remain unchanged. One solution proposed by the present inventor is to separate pixels in the left and right images into corresponding groups, and to adjust differently illuminated image intensities within associated groups of the left and right images. Although this technique for image processing works well with adjusting the brightness of color images, it would be desirable if this technique could be made applicable to adjusting illumination disparities from grayscale stereo image pairs generated from X-ray sources.
Another related problem with conventional techniques for stereoscopic X-ray imaging entails their failure to address the distortion resulting from the toed-in configuration of the X-ray sources. One type of distortion includes depth plane curvature, which could lead to incorrectly perceiving relative object distances on a display. In other applications, depth plane curvature may also disturb image motion. Typically, the distortion of the depth plane results in objects at the corners of the image appearing further away from the viewer (i.e., curved) as opposed to objects that are at the center of the image (i.e., appearing parallel to the surface of the display). Another type of distortion resulting from the toed-in configuration of the X-ray sources is keystone distortion. Accordingly, it is desirable to eliminate such distortions caused by the toed-in configuration of the X-ray sources.
When viewing X-ray stereo images on a monitor or display device, each viewer perceives depth differently. For example, some viewers may perceive depth more easily into the display device than out of the display device, and vice versa. Conventional stereoscopic imaging techniques do not adequately address this drawback of inconsistent depth range perception. Thus, it would be ideal if as many people as possible could view the stereo X-ray images, and especially so if one or more objects within the images are of particular interest to the viewers.
What is needed is a system and method for quickly obtaining three-dimensional (3D) spatial information from two-dimensional stereoscopic binocular pairs of X-ray radiographs, so that objects in the two-dimensional radiographs may be located in three dimensional space. It would be beneficial if some of the stereo methods used to analyze reflective photography of images could be applied to the analysis of projected pairs of grayscale images, for example, using one or more X-ray imaging devices or by using other stereo imaging techniques. It would also be desirable to have a stereo digital three-dimensional imaging system for enabling fast image processing and display, and quick and accurate correction of distortion appearing in the two-dimensional pairs of radiographs, namely resolving illumination disparities amongst the pairs, depth plane curvature, keystone distortion, and inconsistent depth range perception.